Method and system for tone inverting of residual layer tolerant imprint lithography

ABSTRACT

A method (and apparatus) of imprint lithography, includes imprinting, via a patterned mask, a pattern into a resist layer on a substrate, and overlaying a cladding layer over the imprinted resist layer. A portion of the cladding layer is used as a hard mask for a subsequent processing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus forlithography, and more particularly to a method and apparatus for imprintresidual layer management.

2. Description of the Related Art

Imprint lithography describes a class of lithographic methods in which aflat mold 10 (e.g., a transparent mold or template) is pressed into aliquid polymer (resist) 11 on a flat substrate 12, as shown in FIG. 1A.

Then, as shown in FIG. 1B, the polymer 11 is cured by exposure to light13 (e.g., in the case where a transparent mold is used) or heat.

Thereafter, as shown in FIG. 1C, the mold 10 is removed leaving behindan impression of the features of the mold 10 in the polymer 11. Inpractice, the mold 10 is typically flat with fine depth features etchedin its surface. In cases of practical interest, these features can havedimensions that range from many microns to nanometers. The intent isusually to transfer the relief pattern 15 in the polymer resist 11 intothe substrate material using an etch process.

FIG. 1D shows the residual layer 16 of the polymer resist 11 afteretching to expose the surface of the substrate 12.

It is desirable to press the mold into the resist such that very littleresist (e.g., on the order of about 50% of the feature height or less)remains between the unfeatured portions of the mask and the substrate12. Often as little as 40 nm thickness or less is desirable.

Further, it is essential to the subsequent etch process that thisresidual layer 16 thickness be extremely uniform. Practically, this isextremely difficult to achieve due to the viscosity of the resist,flatness and flexibility of the mold 10 and substrate 12 and particulatecontamination.

This results in the need to use ultra-clean, extremely precise, slow andcostly methods to perform lithography at the micron to nanometer scalesthat are of interest. Considerable cost and effort must be expended todevelop systems that reduce and homogenize the residual layer.

Thus, a large challenge in the conventional techniques is achieving asuitable ratio of the thickness of the residual layer to the thicknessof the feature, and thus such ratio typically dictates the processwindow. Practically, obtaining the residual layer extremely thin isdifficult to achieve, as mentioned above.

Indeed, one can imagine that the mask is 1 centimeter (or up to manyinches) square and that the resist is not very viscous, but has a finiteviscosity, pressing the same to 50 nanometers is difficult to perform.That is, as the residual layer becomes thinner and thinner, the viscousshear force increases accordingly and eventually a large amount of forceis required (for pressing) to achieve the desired thinness. This forcehas the tendency to warp the mask and substrate causing furtherinhomogeneities. Finally, as the residual layer thins, more time isrequired for it to move out of the way.

Additionally, as mentioned above, uniformity is problematic since if onedesires a residual layer thickness of 50 nanometers nominally, one wants50 nanometer thickness everywhere and this requires that the mask beperfect, and that the surface and resist must be particle-free. Anyparticulate contamination that is larger than the desired residualthickness will cause local distortions of the mask and substrate. Thesedistortions result in a final print defect that is larger in size thanthe original particle that caused it.

SUMMARY OF THE INVENTION

In view of the foregoing and other exemplary problems, drawbacks, anddisadvantages of the conventional methods and structures, an exemplaryfeature of the present invention is to provide a method and structurefor imprint lithography, and more particularly for imprint residuallayer management.

Another exemplary feature of the present invention is to provide amethod (and system) of imprint lithography which uses a system of masksand photoresists that is tolerant of particulate contamination, residuallayer thickness and of uniformity variations and yet produces highresolution/high aspect resist structures.

In a first exemplary aspect of the present invention, a method (andapparatus) of imprint lithography, includes imprinting, via a patternedmask, a pattern into a resist layer on a substrate, and overlaying acladding layer over the imprinted resist layer. A portion of thecladding layer is used as a hard mask for a subsequent processing.

In a second exemplary embodiment of the invention, an imprintlithography method, includes placing a planarizing, cladding layer overan imprinted layer, wherein a portion of the cladding layer is used as ahard mask for a subsequent processing.

In a third exemplary embodiment of the present invention, a system forimprint lithography, includes an imprint applying unit that imprints,via a patterned mask, a pattern into a resist layer on a substrate, anda spin coating device that overlays a cladding layer over the imprintedresist layer. A portion of the cladding layer is used as a hard mask fora subsequent processing.

The exemplary embodiments primarily employ photoresist as theillustrative example. However, it is noted that thermal cure orcatalytic cure-type resists are also equally applicable. In principle,any material that can be imprinted and hardened in place could be used.Practically, the materials of choice are usually a low viscosity polymerthat hardens to a consistency that holds a stable pattern once the maskis removed, and that can be conveniently etched in subsequentprocessing.

With the invention, the photoresist is printed with whatever residuallayer one gets. Thus, the residual layer can be made relatively thick,but can be made highly uniform. In an exemplary embodiment, support padsor “feet” are placed on the photomask. These feet elevate the mask to aknown and defined height over the substrate. The supporting pads performtwo functions. First, they help to ensure that the photoresist has auniform thickness across the printed field. Secondly, they reduce theprobability that a foreign particle can lodge between the mask andsubstrate and deform the gap and corresponding photoresist thickness.The height of the feet is usually chosen to be larger than most of theambient contamination.

Hence, the pattern is printed and then on top of the photoresist whichhas a uniform thickness. A planarizing photoresist (e.g., an etchresistant cladding layer) is spun on top of the photoresist. Theplanarizing photoresist is formed of a material intended to be somewhatselective in terms of its etch resistance, relative to the imprintedmaterial. Then, an etch is performed on the planarizing resist to exposethe tops of the photo resist, and then the process switches to a secondetch gas, to etch the photoresist (but leaves the spun-on materialalone) down to the substrate.

As a result of the selectivity between the cladding layer and theresist, it is possible to obtain very high aspect ratio (up to 10:1,etc.; where the thickness of the residual layer is large in comparisonto the height of the feature) structures which are printed with a highlyvariable residual layer. Thus, the invention is tolerant of residuallayer thickness, particulates, and little variations in uniformity.

There are several implications which the invention recognizes. First,the planarizing resist truly does planarize and additionally the mask(or template) is etched uniformly in most cases. Neither of these is avery unusual or difficult constraint to accommodate. That is, theinvention will tolerate if the template is slightly tilted or slightlybowed. Also, since the residual layer can be made thicker with theinvention, then the time needed to press the template to thin out theresidual layer is reduced significantly. Thus, the invention allows forhigh throughput and high robustness.

As a result, there is no need to use ultra-clean, extremely precise, andcostly methods to perform lithography at the micron to nanometer scalesthat are of interest. Hence, the invention achieves a suitable ratio ofthe thickness of the residual layer to the thickness of the feature.

Thus, with the invention, a simple method of imprint lithography isprovided using a system of masks and photoresists that is tolerant ofparticulate contamination, residual layer thickness and of uniformityvariations and yet produces high resolution/high aspect resiststructures.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other exemplary purposes, aspects and advantages willbe better understood from the following detailed description of anexemplary embodiment of the invention with reference to the drawings, inwhich:

FIGS. 1A-1D illustrate a simple imprint lithography process;

FIGS. 2A-2D illustrate a spin-cladded imprint lithography process;

FIG. 3 illustrates a flowchart of a method 30 according to the presentinvention;

FIG. 4 illustrates cladded resist structures etched to a silicon dioxidesubstrate surface;

FIG. 5A illustrates a structure 50 after printing and a planarizingresist 52 coating over an organic resist 51, and which illustratesspacer feet 53;

FIG. 5B illustrates the structure of FIG. 5A after etching of theplanarizing resist 52;

FIG. 5C illustrates the structure of FIG. 5B after etching of theorganic resist 53; and

FIG. 6 illustrates an imprint lithography system 60 according to thepresent invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 2A-6,there are shown exemplary embodiments of the method and structuresaccording to the present invention.

Exemplary Embodiment

As mentioned above, conventional practice involves using ultra-filteredresist, extremely clean environments and rigid mask and substratesupport to remedy the above problems. These practices are effective, butvery expensive in terms of time and complexity.

The present invention was designed to overcome these and other exemplaryproblems.

Referring now to FIGS. 2A-2D, the present invention provides a simplemethod of imprint lithography using a system of photoresists that istolerant of residual layer thickness and of uniformity variations andyet produces high resolution/high aspect resist structures.

In the inventive method, imprint is performed in the conventional way(e.g., as in FIGS. 1A-1D) in which a substrate 20 includes a polymerresist 21 thereon. The polymer resist 21 will be referred to as the“imprint layer.”

Turning to FIG. 2A, a second polymer resist layer 22 is then applied(e.g., usually spun-on) the top of the imprint layer 21. Layer 22 willbe referred to as the “cladding” or shield layer. It is noted thatspinning is preferably used since spinning provides a large degree ofprecision. Thus, instead of pressing or positioning a template verycarefully to provide precision, preferably spinning a layer is performedto achieve the desired precision. It is further noted that the claddinglayer preferably is of the planarizing type.

The transfer of the imprinted pattern to the substrate 20 is thenperformed in three (3) additional etch steps.

As shown in FIG. 2B, a first etch step (e.g., which is selective to theshield layer 22) partially etches the cladding (shield) layer 22exposing elevated portions of the imprint layer 21. That is, thecladding layer 22 is etched to expose the features in the imprint layer21.

As shown in FIG. 2C, a second etch step, which is selective to theimprint layer 21 and not selective to the shield layer 22, etches theportions of the imprint layer 21 that were exposed in the first etchstep to the substrate 20.

As shown in FIG. 2D, a third etch step etches the substrate 20. That is,in FIG. 2D, the substrate is usually further processed to either etch,deposit or implant material. Illustrated in FIG. 2 d is the case wherethe resist pattern is transferred to the substrate 20 by etching.

FIG. 3 illustrates a flowchart of a method 30 according to the presentinvention. Specifically, in step 31, the imprint resist is applied to asubstrate.

In step 32, the pattern is imprinted into the resist by pressing themask into the resist and against the substrate, curing the resist andremoving the mask from the substrate.

In step 33, the cladding layer is applied to the top of the resist byspinning or another appropriate method.

In step 34, the imprinted pattern is inverted and the top of thecladding layer is etched until the elevated portions of the resist layerare exposed.

In step 35, a second etch step, which is selective to the imprint layerand not selective to the shield layer, etches the portions of theimprint layer that were exposed in the first etch step, to thesubstrate.

Finally, in step 36, the substrate is further processed to either etch,deposit or implant material.

Again, it is noted that the invention does not rely on pressing andpositioning of the template to obtain the precision (e.g., in obtaininga uniform film thickness) required to achieve the etch selectivityneeded, the invention achieves its precision by preferably using a thinplanarizing cladding layer that is applied by spinning. That is, it ispossible to spin a layer to very high precision. The inventionadvantageously uses such a spinning process. Spinning on a liquid isvery precise (as opposed to deposition such as evaporation orsputtering) since it relies on surface tension and the spin speed/ratecontrol the uniformity of film thickness.

It is noted that other methods of applying a planarizing cladding layerare possible including vacuum and CVD deposition. Spinning isadvantageous because it is simple, cheap and effective.

In the case of a planarizing photoresist, which is of interest in thepresent invention, the inventors have seen a very highly uniformthickness over the surface of the wafer. Such would not be possible witha deposition process, which would evidence bumps, unevenness, etc.Spinning is also rapidly performed, and can be done on a wafer basis (asopposed to being applied to a die). Thus, throughput is enhanced.Precision in spinning can be adjusted by suitably setting the spin speed(which can be set one time).

Thus, the invention preferably and advantageously uses a spinningprocess, thereby eliminating many variables. The inventionadvantageously uses the property of the planarizing photoresist and thespinning process to obtain the precision which is desirable in theresidual layer. Thus, instead of obtaining the precision from somethingwhich is difficult to perform, the invention obtains the precision fromsomething which is relatively easy to do and control.

In comparison, returning to FIGS. 1A-1D showing the conventional method,deposition would result in bumps over the features. To get the residuallayer both thin and perfectly uniform, many variables must be accountedfor (e.g., “right”) including the surface of the residual layer must beparticle-free, the pressing force must be applied perfectly evenly, themask must be perfectly flat, the substrate must be perfectly flat, andthe pressing must be applied long enough for the residual layer toachieve the required thickness. These are difficult things to do inpractice, especially over large areas. Indeed, pressing perfectly aone-inch blank is extremely problematic and difficult, as well astime-consuming (on the order of a minute). The above variables areproblems/issues which the planarizing photoresist and the spinningprocess overcome (and which may take on the order of one second). Asshown in the photograph of FIG. 4, the process is illustrated in anactual photoresist at the point where the substrate surface is exposed.

The exemplary embodiment of the present invention includes two exemplaryresist types. A first type is a cross-linked organic acrylate which isreferred to as the “imprint resist”. A second type is a silatedplanarizing resist which will be referred to as the “shield resist” (orcladding layer resist).

In experimental testing conducted by the present inventors, during theimprint portion of an exemplary operation of the inventive process, aquartz mold (containing the etched features to be transferred on itssurface) was pressed into approximately 100 ml of imprint resist thathas been placed on the surface of the sample. The sample was customarilypre-treated with a commercial adhesion promoter SIA0200.0 or(3-Acryloxypropyl)trimethoxysilane and the mold was pre-treated with acommercial release agent SIT9174.0 ortridecafluoro-1,1,2,2-tetrahydroctyl trichlorosilane.

The mold was pressed against the sample and exposed to broadbandradiation from a mercury source of 25 mW/cm² for 5 seconds and the moldwas removed. The sample was then post baked for 5 minutes at 110 C tocomplete the curing process. The mold was pressed with sufficient force(approximately 3 kg) to leave a 200 nm-300 nm thick layer of imprintresist on the sample.

In a second portion of the resist application process, the shield resistwas applied in liquid form and spun on the sample at 3000 rpm for 30seconds and the sample was post baked at 200 C for 2 minutes.

The reactive ion etch (RIE) portion of the process was performed using aone minute etch in a CF₄27 mT, O₂ 3 mT and 30 W plasma. This etched theshield (cladding) resist to expose the imprinted feature tops.

The second portion of the etch process was performed using a 4-minuteetch in an O₂ 30 mT, 30 W plasma. This etched the imprint resist andexposed the surface of the sample. In the case of a silicon dioxidesample, the former etch plasma that includes CF₄ can be used to etchinto the sample, or in the case of a different material an alternativeetch plasma can be used.

Imprint Resist Formulation:

1,3 butanediol diacrylate 97% mole fraction

2-Hydroxy-2-methyl-1-phenyl-propa-1-one (Ciba Darocure 4265) (initiator)3% mole fraction

5 sec expose at 25 mW/cm2

110 C @ 1.0 minute bake

Shield Resist Formulation:

SiArc SHB-A470 Lot TUO 401215 (Shin Etsu Corp.)

3000 RPM spin @ 30 sec

200 C @ 2.0 min bake

As evident from the above description, an exemplary problem solved bythe present invention is the reduction in sensitivity to the residuallayer thickness as described above.

The spun-on shield layer achieves the precision and uniformity necessaryto reliably use a timed etch to expose the imprint features which canthen be selectively etched to high aspect. The net gain for the processengineer is a dramatic increase in process window for a very simpleprocess.

In contrast, the conventional imprint process is highly sensitive tocontamination, deformation of the sample and mold and the viscosity ofthe resist. The latter requires long periods of time to press theresidual layer to the very thin layer required, and is thus a ratelimiting factor to the conventional process. The inventive process canoperate with a relatively thick residual layer (200 nm-300 nm) andtolerates comparable thickness variations, thus avoiding problems withthickness variations.

A secondary benefit of the inventive process is that the kerf (areabetween the chips) is protected by the shield layer. Under ordinaryimprint processing, the kerf is unprotected and exposed to the etchprocess.

Hence, in the inventive method, several modifications are introduced tothe conventional imprint lithographic method.

The imprint mask may be fabricated or patterned by conventional methods(optical or e beam lithography). Masks are usually created by patterningrelief structures in the surface of flat quartz or sapphire substrates.

Then, the mask may be modified by placing a small number of supportspacers (feet) on the mask surface in non critical (unpatterned)regions. These feet may be round flat features approximately 100 micronsin diameter and placed on the mask contact surface. A variety of methodsand materials can be used to place the spacers. FIG. 5A illustrates astructure 50 after printing and applying a planarizing resist coating 52over an organic resist 51, and which illustrates spacer feet 53. FIG. 5Billustrates the structure of FIG. 5A after etching of the planarizingresist 52. FIG. 5C illustrates the structure of FIG. 5B after etching ofthe organic resist 51. FIGS. 5A-5C illustrate structures similar to thatshown in FIGS. 2A-2D, with the exception of the feet 53.

An exemplary method of placing spacers is to evaporate silicon dioxidethrough a contact molybdenum mask. The thickness of the spacer feet ischosen to be relatively large compared to ambient contamination, but onthe order of 3X-5X the minimum feature size being printed. Placement ofthese features on the mask is chosen such that they do not interferewith critical mask features. On a typical 25 mm×25 mm mask,approximately 9 spacer feet may be applied. The lateral shape of thefeet is not critical. They should simply support whatever mechanicalload is applied to the mask. Typically deposited are 100 micron-wide,300 nanometer-thick round silicon dioxide spacers.

That is, an exemplary embodiment of this invention includes a 25 mm×25mm quartz imprint mask configured with nine 100 micron diameter circularSiO₂ feet arranged in a 3×3 array across the mask. The height of thefeet is approximately 300 nm.

These spacer feet benefit in several respects. First by effectivelythickening the residual layer, the resist more easily flows and reachesequilibrium.

Second, any particulate contamination present on the substrate or in theresist is significantly less likely to influence the placement andparallelism of the mask. The resulting residual layer is thick but veryuniform. Thus, the use of feet on the photomask reduces dramatically theprobability that a large particle or slight force imbalances on mask orsubstrate will cause large residual layer variations due to mechanicaldistortion.

Thus, another alternative embodiment of the invention relates to theprovision or presence of the feet which are helpful to the presentinvention. The feet leave their imprint in the resist. Theirfunctionality is fulfilled once the resist is hardened, but before thespin coating has been applied. The feet guarantee a uniform constantresist thickness. The feet can be any size and the spacing between feetcan be tailored according to the designer's requirements and constraintsto assure that bow and flatness requirements are met according to theapplied load. While the feet may be 10-100 microns thick, there is norequirement to do so. The feet merely should be thick enough to supportthe load put upon them and achieve the design spacing of the mask andsubstrate.

It is further noted that numerous uses and applications for the feetincluding zone plating (e.g., one way of making a lens), immersionlithography, microplating tools, pattern writing, etc. The feet can beapplied to anything in which it is desired to stabilize a distancebetween two plates.

Hence, as shown in FIGS. 2A-2D, imprint is performed in the conventionalway in a polymer resist which is referred to as the “imprint layer.” Asecond polymer resist layer is then applied (usually spun on) the top ofthe imprint layer which is referred to as the “cladding” or “shield”layer. This shield layer takes advantage of the uniformity that isinherent in the spin process.

At this stage of the process, both a uniform known thickness residuallayer and a uniform known thickness cladding or shield layer exist. Theshield layer is chosen to be selective to a different etch process thanthe base resist layer.

The transfer of the imprinted pattern to the substrate is then performedin three additional etch steps. The first etch step (which is selectiveto the shield layer) partially etches the shield layer exposing elevatedportions of the imprint layer. The second etch step which is selectiveto the imprint layer and not selective to the shield layer etches theportions of the imprint layer that were exposed in the first etch stepto the substrate. The third etch step etches to the substrate. Thesesteps are illustrated in FIGS. 2A-2D described above.

It is noted that many variations of the present invention can beundertaken and are well within the scope of the present invention. Forexample, the shield layer can be applied by other methods (e.g.,evaporation, sputtering, imprinting, etc., or a combination thereof).The thickness of the shield layer can be additionally tuned byconventional Chemical-Mechanical Polishing (CMP).

FIG. 6 illustrates an imprint lithography system 60 according to thepresent invention, which includes an imprint applying unit 61 forapplying the imprint layer to a substrate.

A spin coating device 62 applies a second polymer resist layer (claddinglayer) to the imprint layer.

A transfer device 63 transfers the imprinted pattern to the substrate byetching, and specifically a first etch step, which is selective to thecladding layer, partially etches the cladding layer exposing elevatedportions of the imprint layer. Then, the cladding layer is etched toexpose the features in the imprint layer. Thereafter, a second etchstep, which is selective to the imprint layer and not selective to thecladding layer, etches the portions of the imprint layer that wereexposed in the first etch step, to the substrate. The same transferdevice 63 may be used for all etches or dedicated transfer devices maybe employed for each etch.

A further processing device 64 is provided to further process thesubstrate and may take the form of either an etch, a deposition, or animplantation material device. A controller 65 may be employed to controlthe entire process

While the invention has been described in terms of several exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Further, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

1. A method of imprint lithography, comprising: imprinting, via apatterned mask, a pattern into a resist layer on a substrate; andoverlaying a cladding layer over said imprinted resist layer, wherein aportion of the cladding layer is used as a hard mask for a subsequentprocessing, wherein a transfer of the imprinted pattern is performed ina plurality of etches, wherein a first etch, which is selective to thecladding layer, partially etches the cladding layer to expose elevatedportions of the imprint layer, and wherein said first etch comprises oneof a reactive ion etch, sputter etch, plasma etch, wet chemical etch anda chemical downstream etch.
 2. The method of claim 1, where saidoverlaying comprises spinning-on said cladding layer on top of saidimprinted resist layer.
 3. The method of claim 1, wherein saidoverlaying comprises depositing the cladding layer by evaporation. 4.The method of claim 1, wherein said overlaying comprises depositing thecladding layer by sputtering.
 5. The method of claim 1, furthercomprising: adjusting a thickness of the cladding layer via chemicalmechanical polishing (CMP).
 6. The method of claim 1, wherein saidcladding layer comprises a planarizing cladding layer.
 7. The method ofclaim 1, wherein a second etch, which is selective to the imprint layerand not selective to the cladding layer, etches the portions of theimprint layer that were exposed in the first etch to a substrate undersaid cladding layer, and wherein said second etch comprises one of areactive ion etch, sputter etch, plasma etch, a wet chemical etch and achemical downstream etch.
 8. The method of claim 7, wherein a thirdprocess modifies the substrate, said third process comprising a furtherprocessing comprising one of an etch, a deposit, and an implanting of amaterial.
 9. The method of claim 1, wherein said imprint resistcomprises a cross-linked organic acrylate, and wherein said claddinglayer comprises a silated planarizing resist.
 10. The method of claim 1,wherein the cladding layer is applied in a liquid form.
 11. The methodof claim 1, wherein said first etch comprises one of a reactive ion etch(RIE) and an isotropic etch, thereby to etch the cladding layer toexpose tops of imprinted features.
 12. The method of claim 7, whereinsaid second etch comprises a deeper etch than said first etch.
 13. Themethod of claim 1, further comprising: supporting said patterned mask bya plurality of feet.
 14. The method of claim 13, wherein said feetcomprise support spacers placed on the mask surface in unpatternedregions thereof.
 15. The method of claim 13, wherein said feet areplaced by one of evaporating and sputtering a mechanically stablematerial through a mask, a thickness of the feet being with in a rangeof about 3 to about 5 times a minimum feature size being printed.
 16. Amethod of imprint lithography, comprising: imprinting, via a patternedmask, a pattern into a resist layer on a substrate; overlaying acladding layer over said imprinted resist layer; and supporting saidpatterned mask by a plurality of feet, wherein a portion of the claddinglayer is used as a hard mask for a subsequent processing, wherein saidfeet comprise one of silicon dioxide, silicon nitride and amorphoussilicon, wherein said feet are placed by evaporation through a patternedmask, and wherein said mask comprises at least one of molybdenum, steel,copper and aluminum.